Two techniques are used to obtain permanent reshaping of the hair. They are based on cleavage of the disulfide covalent bonds present in keratin (cystine):                the first consists, in a first stage, in performing this opening of the disulfide bonds using a composition containing a reducing agent, and then after, preferably, having rinsed the hair, in reconstituting, in a second stage, the said disulfide bonds by applying to the hair, that has been placed under tension beforehand by means of rollers or the like or shaped or smoothed out by other means, an oxidizing composition also known as a fixer, so as to give the head of hair the desired shape. This technique makes it possible either to make the hair wavy or to relax it, uncurl it or smooth it out;        the second consists in performing a “lanthionization” operation, using a composition containing a base belonging to the hydroxide family. It leads to the replacement of the disulfide bonds (—CH2-S—S—CH2-) with lanthionine bonds (—CH2-S—CH2-). This lanthionization operation involves two consecutive chemical reactions:        The first reaction consists of a beta-elimination on cystine brought about by a hydroxide ion, leading to the cleavage of this bond and to the formation of dehydroalanine.        
                The second reaction is a reaction of the dehydroalanine with a thiol group. Specifically, the double bond of the dehydroalanine formed is a reactive double bond. It can react with the thiol group of the cysteine residue that has been released to form a new bond known as a lanthionine bridge or bond or residue.        

Relative to the first technique using a reducing agent, this lanthionization technique does not require a fixing step, since the formation of the lanthionine bridges is irreversible. It is thus performed in a single step and makes it possible either to make the hair wavy or to relax it, uncurl it or smooth it out. However, it is mainly used for relaxing naturally frizzy hair.
For the first technique, the reducing compositions generally used for the first step of a permanent-waving or relaxing operation contain thiols, sulfites or bisulfites as reducing agent. These agents are generally used in essentially aqueous medium at concentrations of between 0.5 and 1M to obtain good opening of the disulfide bonds. Among the thiols, those commonly used are thioglycolic acid, cysteamine, glyceryl monothioglycolate, thiolactic acid and cysteine. Thioglycolic acid is particularly effective for reducing the disulfide bonds of keratin at alkaline pH, especially in the form of ammonium thioglycolate, and is the product most widely used in permanent-waving (“hair waving”). However, it has been found that thioglycolic acid must be used in a sufficiently basic medium (in practice at a pH of between 8.5 and 9.5) if it is desired to obtain curliness of satisfactory intensity. Besides the drawback of giving off an unpleasant odour requiring the use of more or less efficient fragrances to mask the odours, the use of a thiol at alkaline pH also leads to degradation of the fibre and most particularly to impairment of artificial colorations.
Sulfites or bisulfites are mainly used for relaxing. They have similar drawbacks to thiols, with less efficacy.
Thiols and sulfites (or bisulfites) also have the drawback of having poor stability in aqueous solution.
In general, the durability of the reshaping effects obtained with thiols and sulfites by reduction of disulfides followed by fixing is judged to be very much inferior to that which may be obtained via the lanthionization technique.
For the second technique, the compositions generally used to perform the lanthionization contain as base a hydroxide such as sodium hydroxide, guanidinium hydroxide or lithium hydroxide. These lanthionization active agents, which allow the disulfide bonds to be opened via a beta-elimination mechanism, are generally used as a water-oil emulsion at concentrations of between 0.4 and 0.6M, by leaving them to act generally for 10 to 15 minutes at room temperature. Sodium hydroxide remains the agent most widely used. Guanidinium hydroxide is now the preferred compound for many compositions. These two hydroxides, sodium hydroxide and guanidinium hydroxide, are the two main agents used for relaxing or uncurling naturally frizzy hair. They have several advantages over ammonium thioglycolate and sulfites, in particular an absence of unpleasant odour, the fact that only one implementation step is required (shorter treatment time), and a much greater durability and efficacy of the reshaping of the hair.
However, these hydroxides have the major drawback of being caustic. This causticity affects the scalp by causing irritation, which may occasionally be severe. This may partially be remedied by first applying to the scalp a fatty protective cream often referred to as a “base” or “base cream”, the word “base” in this instance not having the meaning of a basic agent in the chemical sense. When the protective cream is combined with hydroxide in a single composition, it is generally known as a “no-base” cream, as opposed to the above name. This “no-base” technology is preferred.
The causticity of hydroxides also affects the condition of the hair by firstly giving it a coarse feel and secondly making it much more fragile, this fragility possibly going as far as flaking or even breaking, or even the dissolution of the hair if the treatment is prolonged. In certain cases, hydroxides also cause decolorization of the natural colour of the hair.
Formulations containing sodium hydroxide are generally known as “lye relaxers” and those not containing it are known as “no-lye relaxers”.
The main “no-lye” relaxing formulations use guanidinium hydroxide. Since guanidinium hydroxide is unstable, it is generated extemporaneously by mixing guanidine carbonate and a source of very sparingly soluble hydroxide such as calcium hydroxide. The reaction between these two compounds leads to the formation of guanidinium hydroxide and calcium carbonate, which precipitates in the composition. The presence of this precipitate makes the final rinsing of the hair much more difficult and leaves on the hair and the scalp mineral particles that give it a coarse feel and an unattractive appearance resembling dandruff. The recent success of guanidinium hydroxide (“no-lye”) in the face of sodium hydroxide (“lye”) appears to arise from better relaxing efficacy and better skin tolerance. However, these technologies using bases of the hydroxide family remain very aggressive for the hair and the scalp and require very strict control of the application time to avoid excessive irritation and impairment of the hair, which may go as far as breaking of the hair. This aggressiveness arising from the causticity of hydroxides is just reason for these hair lanthionization compositions not being used for permanent-waving (“hair waving”), but being reserved for relaxing (“hair straightening” or “hair relaxing”).
Furthermore, hydroxides are known to be good agents for hydrolysing amide functions (cf. for example March's Advanced Organic Chemistry, 5 ed., Wiley Interscience, New York, “Hydrolysis of Amides” pages 474 et seq.), which thus lead to cleavage of the peptide bonds via direct nucleophilic attack. It is thus probable that the impairments observed on the hair and keratin materials in the broad sense are largely due to partial hydrolysis of the amide bonds of keratin.
There is thus a real need for relaxing compositions that are markedly less aggressive to the hair.
Various studies have been conducted in order to overcome the drawbacks of both reducing agents (first technique) and/or of hydroxides (second technique).
Thus, to replace thioglycolic acid, numerous reducing agents have been proposed, but thioglycolic acid in its ammonium thioglycolate form remains both the compound of reference and the compound most widely used in cosmetic formulations, both for shaping and for straightening.
It has also been proposed in numerous patents to combine common reducing agents (thiols, sulfites or bisulfites) with urea or alkylureas to reduce the irritation and damage caused on the hair both by shaping and by relaxing. Reference will be made, for example, to:                patent application CA 1315204, which describes a composition containing ammonium thioglycolate (5.5-11.5%) and urea or a monoalkylurea (1-3%) for shaping the hair,        U.S. Pat. No. 3,847,165, which describes a composition containing ammonium thioglycolate (1.2-1.4M) and urea (2.0-2.7M) for shaping the hair at acidic pH,        patent application NL 6 410 355, which describes a composition containing a sulfite (0.8-1.5M) and urea (0.6-3.0M) for shaping and relaxing the hair,        patent application JP 2000/229 819, which describes a composition containing a sulfite or bisulfite (0.5-15%), urea (0.5-15%) and an alcohol (ethanol and/or isopropanol, 1-30%) for shaping and relaxing the hair.        
It has also been proposed in numerous patents to combine hydroxides, serving as lanthionization active agent, with certain additives generally serving to protect the hair. Mention will be made, for example, of:                patent application WO 2002/003 937, which describes a composition containing C3-C5 monosaccharides,        patent application WO 2001/064 171, which describes a composition containing complexing agents,        U.S. Pat. No. 5,641,477, which describes a composition containing a hydrogenated starch hydrolysate,        patent application WO 02/085 317, which describes a composition containing organic nucleophiles, which react during the second step with the dehydroalanine formed with hydroxides, to give new bridges.        
Although all these proposals lead to more or less marked improvements, they do not make it possible to significantly reduce the damage caused by the causticity itself of hydroxides.
As indicated above, the use of reducing agents leads to mediocre durability for relaxing or uncurling, and the use of hydroxides, on account of their causticity, limits their use to the field of relaxing.